A monitor lizard ignores the textbook explanations of how it should breathe and instead it whooshes air through its lungs in a one-way flow, just as birds do.

As mammals breathe, air flows like a tide into the lungs and then back out through the same tubes. Birds also move air in and out of their mouths and noses. But inside their lungs, birds route air in one direction only. Lizards, in theory, had tidal lungs like mammals. Rethinking that upsets ideas about lung evolution.

Sensors implanted in the lungs of live savannah monitor lizards (Varanus exanthematicus) showed air blowing one way, says Colleen G. Farmer of the University of Utah in Salt Lake City. Breath moves through the animal’s multichambered lungs from the tail end toward the head. Farmer’s group found air wafting from one chamber to another through walls dotted with openings “like lace curtains,” she says.

Moving water through sections of dissected monitor lungs suggests that the sizes and angles of the lung’s pathways direct flow in a single direction without muscle control, valves or flaps, Farmer and her colleagues report December 11 in Nature.

This demonstration expands recent turmoil surrounding vertebrates’ lung evolution. Before 2010, many biologists treated one-way air flow in lungs as unique to birds, Farmer says. Such flow allows the air-to-blood exchange surfaces to stay thin and fragile, since they don’t have to expand and contract with tidal pumping. Those delicate structures transfer gases more efficiently than the tougher, thicker air sacs of mammalian lungs. So, one theory went, one-way air flow improved efficiency, giving birds greater athletic capacity and helping them to develop flight.

Yet in 2010, Farmer and a colleague published measurements of one-way airflow in an alligator lung (SN: 2/13/10, p. 11). That raised the possibility that a common ancestor of birds and alligators might have already developed one-way lungs. Now, lizards push that possibility some 20 million years deeper in time. A common ancestor of alligators, birds and lizards probably lived about 250 million years ago.

Long-held ideas on what drove the evolution of one-way lungs “have to be wrong,” Farmer says. Such an ancient ancestor would not have evolved flight or self-regulation of body temperature, another explanation for one-way flow, so those factors wouldn’t have been involved.

“It’s always sticky to try and find out what drove evolution,” says Jonathan Codd of the University of Manchester in England. What structures do can be quite different from the function that pushed their evolution in the first place.